1. Field of the Invention
This invention relates to sealing systems for sealing an electrical cable against leakage from ambient. More particularly, the present invention relates to an improved high temperature slip-sealing grommet systems for preventing leakage at splices and cable ends of electrical cables such as heating cables over extended high temperature operating intervals and cool down periods.
2. Introduction to the Invention
The ends of elongate cables such as power cords or heating cables often must be sealed from the ambient in order to provide electrical insulation, environmental protection, leakage from an ambient fluid medium such as moisture or other liquids, and/or mechanical shielding. Many methods and devices have been used to provide a seal, including heat-recoverable tubing, end-caps, or boots; molded adhesive-filled boots; enclosures with gaskets or grommet seals; and wrapped tape. One example of a sealing device for an elongate heating cable is provided in the U.S. Pat. No. 5,792,987 entitled xe2x80x9cSealing Devicexe2x80x9d, by the present inventor and others, the disclosure thereof being incorporated herein by reference.
The prior sealing methods and devices have not been entirely satisfactory, particularly for preventing leakage after the assembly has been subjected to high temperature over an extended operating interval, followed by a cool-down interval. One exemplary system illustrative of the prior art and this problem is the S-150-E trace heating cable splice kit offered commercially by the Raychem HTS division of Tyco Flow Control, a part of Tyco International Ltd. This kit 10 is shown in pertinent part in FIG. 1. Therein an electrical cable such as a trace heating cable is slipped through a pressure plate 12, a spring 14, a rigid compression ring 16, a silicone rubber grommet 18, and a back shim 20 before reaching an enclosed interior space of an enclosure body 22 wherein the cable end terminates or may be spliced to another cable end. The enclosure body 22 includes an annular shoulder projection inset from its opening which engages a peripheral region and thereby retains the back shim 20 against further axial displacement as the assembly is pushed into the enclosure body 22. While present, this projection is not shown in the FIG. 1 illustration. Screws 24 are used to drive the pressure plate inwardly towards the enclosure body 22, which compresses the spring 14 and grommet 18. In this example of the prior art, the spring 14 comprised a crest-to-crest five-turn, half-inch free height spring, such as a Smalley Spirawave(trademark) C087-H3 spring having a theoretical spring rate (lbs/in) of 180. When compressed by force from the spring 14, grommet 18 expands radially against both the heating cable and an interior wall of the enclosure body 22 thereby nominally sealing the heating cable to the grommet 18, and the grommet to the enclosure body 22.
Silicone rubber is widely used as a high temperature elastomer. Although its mechanical properties are not unduly affected by exposure to high temperatures (temperatures above 200xc2x0 C., for example), silicone rubber may adhesively bond at such high temperatures to certain engineering plastics such as polyphenylene sulfide (PPS). If a silicone rubber grommet, such as grommet 18, bonds to an enclosure body 22 formed of PPS, the grommet 18 is no longer free to move in response to applied stress and cannot continue to maintain a seal. This behavior has been noted in the S-150-E trace heating cable splice kit 10 illustrated in FIG. 1.
The process by which the splice kit 10 loses its seal is as follows. First, the seal kit 10 is assembled onto the heating cable and inserted into the enclosure body 22. The completed assembly is then subjected to a high temperature, e.g. in excess of 200xc2x0 C. The assembly components expand in response to the high temperature. The volume thermal expansion coefficient of silicone rubber is about three times that of engineering plastics such as glass-filled PPS over the range of temperatures encountered in service. Consequently, the only direction for the silicone rubber grommet 18 to expand is longitudinally along the enclosure body 22 toward the pressure plate in the FIG. 1 example, because the grommet 18 is constrained longitudinally by the back shim and enclosure body but may move longitudinally toward the pressure plate 12 by further compression of the spring 14.
Over an extended time (hours or days) at the elevated temperature, the silicone rubber of the grommet 18 adhesively bonds to plastic parts like the PPS enclosure body 22, but does not adhere to the fluoropolymer outer jacket of the heating cable. When the assembly is later cooled to room temperature, the silicone rubber grommet 18 shrinks back to its original volume. However, because an outer annular surface of the grommet 18 has bonded to the enclosure body 22, the grommet 18 cannot shrink longitudinally. The only remaining dimension for the material to shrink is radially, and it does so, pulling away from the heating cable outer jacket, and forming a leakage path between the cable jacket and the grommet 18. This unwanted tendency to develop leakage paths has become increasingly acute in the face of modern industry-initiated uniform testing standards which now require testing of heating cables over extended times at temperatures in excess of 200xc2x0 C.
Therefore, a hitherto unsolved need has developed and remains for a heating cable sealing kit which provides an assembly which does not develop leakage paths over time and when subjected to high temperature cycles.
A general object of the present invention is to provide a heating cable sealing kit and assembly for an electrical cable such as a heating cable which does not develop leakage paths over time-extended high temperature cycles and which can be made out of molded plastic and silicone rubber materials which withstand high temperatures.
Another object of the present invention is to provide a high temperature slip-seal grommet system which includes a slip-plane formed at adjacently confronting cone-cup shaped surfaces for more perfectly translating axial force into radial compression force and for permitting relative slippage of an elastomeric grommet along the slip-plane without becoming entirely axially bonded to an enclosure body over high temperature operational cycles and thereby preventing unwanted leakage at an interface with a heating cable jacket in a manner overcoming limitations and drawbacks of prior grommet systems and methods.
One more object of the present invention is to provide a kit of parts for assembly by a craft worker into a sealing system for sealing an end region of an elongate heating cable received through a central longitudinal opening, the kit of parts including a high temperature elastomeric grommet forming a slip-plane relative to a spring-loaded cup-shaped member.
The present invention comprises a seal kit forming a seal assembly for an elongate electrical cable such as a heating cable. The kit essentially includes a shaped elastomeric grommet, several molded plastic parts, and a compression spring. The compression grommet forms a sealing interface between itself and a molded plastic enclosure body and between itself and an outer jacket of a heating cable. Thus, electrically live ends of the cable can be effectively sealed within a sealed interior space of the enclosure body. A cup-shaped surface of a molded plastic part adjacently facing a congruent cone-shaped surface of the elastomeric grommet forms a slip-plane. The compression spring stores mechanical energy which is transferred to compress the elastomeric grommet and to cause it to expand radially against an interior wall of the enclosure body and against an outer jacket of the electrical cable. The cup-shaped surface slips over an outer region of the elastomeric grommet and prevents that region from becoming bonded over a high temperature operating interval to the inside wall of the enclosure body. When the assembly cools down, the elastomeric grommet is thereby permitted to shrink longitudinally by slippage along the slip-plane formed at the cup-shaped surface, thereby maintaining a radial seal against the outer jacket of the electrical cable.
Accordingly, an apparatus is provided for sealing an end region of an elongate cable such as a heating cable. The apparatus receives the heating cable through a central longitudinal opening, and includes an enclosure body of molded plastic material defining at least a first open end region leading to a fully-enclosed interior space for the cable end. The open end region of the body has a spring force stopping structure and defines an interior wall surface. A pressure plate formed of rigid material is releasably secured to cover and close the open end region of the body. A compression spring applies a longitudinal compression force between the pressure plate and the spring force stopping structure. A grommet formed of elastomeric material having a tendency to bond at high temperature with the material comprising the body but not to bond to material comprising an outer jacket of the heating cable, is compressed by the compression force. The grommet includes at least one generally cone-shaped face confronting a cup-shaped face of a structural member within a compression path between the pressure plate and the spring force stopping structure, thereby creating a slip-surface. The cup-shaped geometry of the member at the slip-surface prevents the grommet from becoming bonded over a high temperature operating interval to the inside wall of the body. When the assembly cools down, the elastomeric grommet is thereby permitted to shrink longitudinally by slippage along the slip-plane formed at the cup-shaped surface, thereby maintaining a radial seal against the outer jacket of the electrical cable.
In one aspect of the invention, the compression spring lies between the pressure plate and a rigid molded plastic shim forming the cup-shaped structural member. In another aspect of the present invention, the cup-shaped structural member is a shim of slippery plastic material imposed between the elastomeric grommet and a compression ring within the compression path. In one other embodiment of the invention, the cup-shaped structural member is integrally formed with the spring force stopping structure. In yet another embodiment of the invention, the spring force stopping structure is an end wall of the body opposite the open end, and the compression spring seats directly against the end wall and engages a rigid molded plastic shim forming the cup-shaped structural member.
These and other objects, advantages, aspects and features of the present invention will be more fully understood and appreciated by those skilled in the art upon consideration of the following detailed description of preferred embodiments, presented in conjunction with the accompanying drawings.